A
less cruel (and more
practical - I often don't have ice water on me) method for
awakening someone
unconscious or
asleep, is to rub your
knuckles in a rough way on the
chestplate of the
incapacitated person. This is very
unpleasant, and will make the
victim wake up (if not in a
coma).
The reason all these methods work is that the body experiences discomfort, and seeks to change its current state, which often requires using the brain.
Note that this will often not work in a physically induced coma. However, the icy water in ear method will, as it works through the autonomous nervous system.
(thanks for the extra info, alex.tan :)